Boot to Recovery Mode by holding VOL UP+HOME+POWER. Install ZIP of SuperSU. Reboot phone. Update SuperSU if needed, test if it worked by opening it. It will prompt you to disable KNOX, go ahead and let it. It was successful for me.

via XDA and Androidayos. As always, you follow any of my advice at your own risk! Not responsible for bricked phones etc.

If above doesn’t work for you, you can try L900 CF-Auto Root (flashed via ODIN PDA) as well. I did this prior to the above but it was unsuccessful on its own.